FWU: Add helper functions for accessing FWU metadata

Add weak functions for getting the update index value and dfu
alternate number needed for FWU Multi Bank update
functionality.

The current implementation for getting the update index value is for
platforms with 2 banks. If a platform supports more than 2 banks, it
can implement it's own function. The function to get the dfu alternate
number has been added for platforms with GPT partitioned storage
devices. Platforms with other storage partition scheme need to
implement their own function.

Signed-off-by: Sughosh Ganu <sughosh.ganu@linaro.org>
Reviewed-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
Acked-by: Etienne Carriere <etienne.carriere@linaro.org>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>